What is a Microbe?
A microbe is any living organism that spends its life at a size too tiny to be seen with the naked eye. Microbes include bacteria and archaebacteria, protists, some fungi and even some very tiny animals that are too small to be seen without the aid of a microscope. Viruses and the recently discovered prions are also considered microbes.
The term microbe is short for microorganism, which means small organism. To help people understand the different types of microbes, they are grouped or classified in various ways. Microbes are extremely diverse and represent all the great kingdoms of life, including the animals, plants, fungi, protists and bacteria (see the tree below). In fact, in terms of numbers, most of the diversity of life on Earth is represented by microbes.
Here is an outline of the major groups of organisms:
At the most basic level, all living things can be divided into two major groups of organisms depending on the type of cells they have. Those organisms which contain more complex cells and which have nuclei are called eukaryotes ("true kernal or nucleus"). These organisms include animals, plants, fungi and protists. The other group of organisms are less complex and lack nuclei and are called prokaryotes ("before nucleus"). The prokaryotes are also known as bacteria or as monerans. An amazing recent discovery has shown that the bacteria can be divided into two groups, one of which, the archaebacteria, is more closely related to humans than is the other group of bacteria, the eubacteria, which are discussed below.
All microbes are related to one another and to humans. To see how all forms of life are related, check out the Tree of Life World Wide Web site.
Prokaryotes? contain no nucleus or other cell organelles. Often simple in form, bacteria have a huge range of biochemical diversity, meaning that they can grow on many different compounds.
Prokaryotes are lumped into two groups, the Eubacteria and the Archaea. Although many Eubacteria and Archaea look similar, if you were to look at the molecules that make them, you would find that the Eubacteria differ more from the Archaea than the Archaea differ from the rest of the Eukaryotes, which include plants, animals, fungi and protists.
These include most of the bacteria that people typically think of when they think of bacteria.
The archaea are similar to the eubacteria in size and in many forms, yet they differ in many ways from the eubacteria. Generally, this group of bacteria, also known as archea, thrive in extreme environments, such as hot thermal vents, under conditions with no oxygen or in highly acid environments.
Protists are defined more by what they are not than what they are. They are an extremely diverse group of microbes with a fascinating variety of shapes. Protists include all microscopic organisms that are not bacteria, not animals, not plants, and not fungi. Protists are the microscopic members of the kingdom Protocista.
Fungi are organisms that scientists once confused with plants because they are non-motile, like plants. However, thanks to new research tools, scientists have found that, at the molecular level, the fungi are more like animals than they are like plants. For one thing, fungi cannot synthesize their own food via photosynthesis, like plants do, but instead they feed off of other organisms as do animals.
Plants are organisms that can obtain energy from sunlight. All plants are relatively large, and hence none are generally considered microbes.
Are animals microbes? If it is too tiny to be seen with a naked eye, then it is a microbe. Most animals, like cows, butterflies and earthworms are not microbes. Tiny animals like rotifers and nematodes are considered microbes.